Many thermostatic switches are known which have contacts mounted on one or more flexible strips, with at least one of the flexible strips being formed of or biased by a bimetal material for movement subject to temperature variations. With such constructions, it is possible to provide relatively accurate electrical control responsive to temperature changes.
One particular type of thermostatic switch is the snapaction thermostatic switch. The snap action is accomplished by the use of a formed section in a bimetallic member, the formed section usually being essentially dish-shaped, snapping from a convex to a concave, or a concave to a convex shape when a preset temperature is reached. A contact disposed at the end of the bimetallic member is thus moved into or out of engagement with a mating contact, resulting in a sudden separation of the two contacts.
In order to provide proper calibration of the thermostatic switch, as well as to provide a fulcrum for the moving member, a calibration dimple is formed in a portion of the thermostatic switch which bears against the bimetal member, and particularly against the formed section. Generally, this dimple is part of the top of the casing within which the switch is housed, formed by an indent, a mass of solder, or other means. Alternatively, this dimple could be provided on a contact strip from which the bimetallic member is supported. Because the bimetal member is prevented from bending beyond the fulcrum provided by the dimple, the member is prevented from moving closer to the casing, maintaining the member in contact with the opposed contact until such time as the formed section snaps.
It has been found, however, that thermostatic switches having calibration dimples provide a current leakage path through the dimple, which heats the formed section and causes an instability in the time required to trip the bimetallic member. Since this calibration dimple is usually positioned such that the formed section rests on the dimple, excess current leakage could also damage the formed section. Since the cross-sectional area of the dimple is quite small, a concentrated current flow through the dimple could produce arcing at that point which could burn a hole in the formed section.